Coherent consolidation of trillions of nucleations for mono-atom step-level flat surfaces

Abstract

Constructing a mono-atom step-level ultra-flat material surface is challenging, especially for thin films, because it is prohibitively difficult for trillions of clusters to coherently merge. Even though a rough metal surface, as well as the scattering of carriers at grain boundaries, limits electron transport and obscures their intrinsic properties, the importance of the flat surface has not been emphasised sufficiently. In this study, we describe in detail the initial growth of copper thin films required for mono-atom step-level flat surfaces (MSFSs). Deposition using atomic sputtering epitaxy leads to the coherent merging of trillions of islands into a coplanar layer, eventually forming an MSFS, for which the key factor is suggested to be the individual deposition of single atoms.Theoretical calculations support that single sputtered atoms ensure the formation of highly aligned nanodroplets and help them to merge into a coplanar layer. The realisation of the ultra-flat surfaces is expected to greatly assist efforts to improve quantum behaviour by increasing the coherency of electrons. Keywords: Coherent merging; coplanar layer; initial growth stages; mono-atom step-level flat surface; single-crystal thin films Ultrathin metal films are indispensable in modern electronics and nanotechnology 1-3 . During the past few decades, conventional metals have been studied extensively because the performance of metal-based devices is intimately related to their physical properties. [4][5][6] Efforts to produce single-crystal (SC) copper (Cu) from Cu foil have been driven by competition 7,8 and interest in their nanocrystalline nature and potential applications in large two-dimensional (2D) components consisting of materials such as graphene and hexagonal boron nitride (h-BN). [9][10][11] However, despite the importance of metal thin-film flatness, there have been few reports because it is challenging to control flatness. Since the contact between the metal electrode and the semiconductor material decisively affects the properties of electronic and optoelectronic devices, 12,13 a flat metal surface is proposed as a good solution to reduce contact resistance. The motion of electrons without scattering at surfaces and grain boundaries can also affect the carrier transport properties. 14 Single-crystal Cu thin films (SCCFs) on sapphire have recently been reported, and the formation of twin boundaries (TBs) have been investigated intensively. [15][16][17] Two orientations (ORs) adjacent to a TB are rotated by a certain angle in-plane and satisfy the symmetry operation exactly, while two ORs adjacent to a grain boundary (GB) are tilted both in out-of-plane and in-plane directions. However, in view of electronic motion, a much clearer distinction between TB and GB must be followed by precise microscopic analysis. Twin